1. Field of the Invention
The present invention relates to an apparatus and a method of measuring a viscoelastic characteristic value such as Young's modulus, a loss factor, and the like of a viscoelastic material such as synthetic resin, crosslinked rubber, and the like. More particularly, the present invention is intended to measure the viscoelastic characteristic value of the viscoelastic material accurately by using a so-called split Hopkinson's bar.
2. Description of the Related Art
In recent years, to analyze the deformation and behavior of an object to which an impact is applied, simulation is used rather than measurement. In the simulation, it is necessary to perform substitutions of the viscoelastic characteristic value (parameter) such as the Young's modulus, the loss factor, and the like of the object. The parameter is classified into a static parameter and a dynamic parameter. Because the deformation and behavior of the object is dynamic, the dynamic parameter measured in a state close to the deformation and behavior is effective for the simulation. The measurement of the dynamic parameter is also important for apprehending the characteristic of the object.
As means for measuring the dynamic parameter, an apparatus using the split Hopkinson's bar is known. The split Hopkinson's bar is used in the field of metal material (see page 173–183 of “Impact Engineering” published by Nikkan Kogyo Newspaper Ltd. on Oct. 28, 1989) or the like. In the apparatus using the split Hopkinson's bar, an impact bar, an input bar, and an output bar all made of metal are arranged in a straight line; a specimen is held between the rear end of the input bar and the front end of the output bar; and a strain gauge is installed on each of the input bar and the output bar.
In measuring the viscoelastic characteristic of the specimen, the front end of the input bar is hit by the impact bar. A strain wave generated at the impact time propagates to the specimen and the output bar from the input bar. The following three waves are measured with gauges installed on the input bar and the output bar to compute the viscoelastic characteristic value of the specimen: An incident strain wave progressing in the input bar to its rear end, a reflected strain wave reflected from the rear end of the input bar and progressing to its front end, and a transmitted strain wave transmitted from the rear end of the input bar to the rear end of the output bar through the specimen.
It is to be noted that in the description made below the incident strain wave, the reflected strain wave, and the transmitted strain wave are abbreviated as a “strain wave” as necessary and that the input bar and the output bar are abbreviated as a “stress bar” as necessary.
The measuring apparatus is capable of measuring the characteristic value of a metal material but has difficulty in measuring the viscoelastic characteristic value of a polymer such as synthetic resin, crosslinked rubber, and the like. When the specimen is made of the polymer, there is a large difference between the characteristic impedance of the specimen and that of the stress bar made of metal. This is because it is difficult to pick up the strain wave propagating in the input bar, the specimen, and the output bar correctly. In measuring the viscoelastic characteristic value of the polymer, it is necessary to select the stress bar having a small difference between the characteristic impedance thereof and that of the specimen.
A viscoelastic characteristic value-measuring apparatus using the stress bar made of the polymer instead of the metal bar is disclosed by Nakagawa of Hiroshima University and others on pages 25–29 of lecture thesis of 16th series of Chugoku Branch of Japan Design Engineering Society Association. Unlike the stress bar made of metal, the strain wave is attenuated greatly in the stress bar made of the polymer. For example, the incident strain wave progressing in the input bar to the specimen is attenuated before it reaches the rear end of the input bar after it is measured with a strain gauge installed on the input bar. Thus, it is impossible to correctly assume the incident strain wave at the rear end of the input bar. Similarly, it is impossible to correctly assume the reflected strain wave reflected from the rear end of the input bar and progressing to the front end thereof and the transmitted strain wave transmitted to the output bar from the rear end of the specimen.
In the viscoelastic characteristic value-measuring apparatus disclosed by Nakagawa and others, two strain gauges are installed on each of the input bar and the output bar to solve the problem of the damp of the stress bar made of the polymer. That is, a transmission function is derived from the incident strain wave, the reflected strain wave, and the transmitted strain wave measured with the two strain gauges. From the transmission function, the strain amount of each of the incident strain wave at the rear end of the input bar, the reflected strain wave at the rear end of the input bar, and the transmitted strain wave at the front end of the output bar are estimated. The viscoelastic characteristic value-measuring apparatus is capable of measuring the viscoelastic characteristic value of the specimen when the specimen deforms greatly at high speed (maximum strain speed: 500–8000 per second) and in a large amount (maximum deformation amount is in the range from 1% to 30%).
The viscoelastic characteristic value-measuring apparatus is capable of correctly measuring the viscoelastic characteristic value of a comparatively hard polymer, but has a large error in measuring the viscoelastic characteristic value of a comparatively soft viscoelastic material. The error is attributed to the fact that as the specimen becomes softer, the difference between the progress speed of the strain wave in the specimen and that thereof in the input bar and the output bar disposed forward and rearward from the specimen becomes increasingly large.
That is, in the case of the specimen made of the comparatively soft viscoelastic material, the progress speed of the strain wave is higher in the input bar than in the specimen. The strain wave is reflected by its rear end. When the input bar is short, a first reflected strain wave (to be measured with the strain gauge installed on the input bar) reflected from the rear end of the input bar progresses to the front end thereof, reaches its front end at which the first reflected strain wave is reflected (second reflected strain wave). Thus, when the input bar is short, it is difficult to measure a correct strain amount of the reflected strain wave, because the second reflected strain wave is also measured with the strain gauge installed on the input bar, with the first and second reflected strain wave interfering with each other. Accordingly, it is necessary to space the strain gauge at an appropriate interval from the front end of the input bar to damp the second reflected strain wave. It is also necessary to space the strain gauge for measuring the incident strain wave and the strain gauge for measuring the first reflected strain wave at a required interval because near the rear end of the input bar, the incident strain wave and the first reflected strain wave interfere with each other. For this reason, the input bar is required to be long.
On the other hand, because the strain gauge installed on the output bar measures only the transmitted strain wave, it can be installed near the front end of the output bar. Even though the output bar is short, the strain gauge is distant from the rear end of the output bar. Thus, the reflected strain wave is not measured with the strain gauge nor interferes with the transmitted strain wave.
However, in the conventional viscoelastic characteristic value-measuring apparatus, the length of the output bar is set equal to that of the input bar. As the output bar becomes long, the output bar becomes increasingly flexible. Consequently, the transmitted strain wave having a small strain amount is measured under the influence of noise. Further, the long output bar causes the viscoelastic characteristic value-measuring apparatus to be large.